FRIDAY, Dec. 12, 2014 (HealthDay News) -- A large, new study should reassure the millions of American women who have migraine: The debilitating headaches don't raise the risk for breast cancer.

"There is no association between migraine and breast cancer risk," said lead researcher Rulla Tamimi, an associate professor of medicine at Harvard Medical School. "There is no positive association, so there is no reason for concern, and there is no protective effect either."

About 18 percent of American women and 6 percent of men suffer from migraine, according to the Migraine Research Foundation. Migraine is a syndrome involving severe headaches that are often accompanied by visual disturbances, nausea, vomiting and sensitivity to sound and light. Attacks can last from four to 72 hours.

The idea that migraine and breast cancer might be connected arose because both involve sex hormone levels, Tamimi explained.

"We don't really understand the cause of migraines, but there has been a suggestion that they may be triggered by hormone levels," she said. "We know that hormone levels are also associated with breast cancer, so perhaps there could be a link between migraine and breast cancer."

But the report -- based on more than 100,000 women -- observed no association between migraine and breast cancer or migraine and female sex hormones, Tamimi said. The findings were published Dec. 12 in the Journal of the National Cancer Institute.

The research data, which included analysis of four other studies, hinted that migraine might even lower the risk for breast cancer, but that appeared to be a result of study design and not necessarily a real link, the researchers said.

Dr. Mark Green, a professor of neurology at Mount Sinai Hospital in New York City, said that the idea that migraines might be protective against breast cancer is intriguing.

"There is a suggestion that migraine is linked to a lower incidence of breast cancer. However, the numbers were small, and it's not appropriate to suggest that yet, but it's an interesting idea," said Green, who was not involved with the study.

Dr. Stephanie Bernik, chief of surgical oncology at Lenox Hill Hospital in New York City, discussed the study's significance: "In the ongoing quest to find out what contributes to the causes of cancer, sometimes a negative association can also shed light on the subject," she said. "This study will give relief to many migraine sufferers, and give them one less thing to worry about."

The study relied on data on over 115,000 women who took part in the Nurses' Health Study II, about 18,000 of whom suffered from migraine. Over 20 years of follow-up, no association with breast cancer was observed.

Tamimi's team also looked at the levels of sex hormones of about 2,000 premenopausal women and found no link between hormone levels and migraines.